Uses k_size as an signed integer buf_incrwritepos() and mp_to_unsigned_bin() functions use k_size as signed integer argument. k_size is also used in an assertion that compared it to curve_size which is a signed long. Only buf_getwriteptr() is using k_size as unsigned. So it safe to use it as signed.

256-bits (k = 36113) = 115792089237316195423570985008687907853269984665640564039457584007913129603823
512-bits (k = 38117) = 13407807929942597099574024998205846127479365820592393377723561443721764030073546976801874298166903427690031858186486050853753882811946569946433649006045979